This invention relates generally to a safety device for a steam turbine wheel as part of a turbine rotor, and particularly, to a member or finger disposed at the bore of the wheel's hub section locking into a slot on the shaft of the rotor thereby preventing the wheel from rotating relative to the shaft when the interference shrink fit between the wheel and shaft loosens.
Some steam turbines utilize such large rotors, that the turbine wheels, which carry the turbine blades at their radially outer most portions, are not an integral part of the shaft of the rotor. The radial dimensions of such turbine rotors are on the order of seven or eight feet excluding the turbine blade dimension. It is well known in the art that such large rotors experience great amounts of stresses, due to their size and due to the quality and quantity of steam affecting their buckets. In addition to the turbine blades, each wheel includes a hub section generally disposed at its radially inward portion.
Each hub section has a bore therethrough. The wheel is secured to the shaft of the rotor by an interference shrink fit between the radially inner surface of the hub section and the corresponding surface of the shaft. During normal turbine operations, this interference fit prevents rotation of the wheel relative to the shaft.
Due to the large size of the steam turbine rotor, the initial cost of the mechanism, and the operating costs of the device, it is required that the turbine blades be maintained at a substantially fixed positions relative to the shaft. This requirement should be met during all turbine operations, including normal but non-steady state conditions such as overspeed and undesirable thermal transient periods. This requirement is of greater importance when the steam turbine rotor is acted upon by steam generated in a nuclear boiler. To insure that the turbine wheels do not rotate relative to the shaft during such turbine operations, a safety device or a redundant locking means is incorporated into the wheel, such as the bore through the hub section of the wheel.
It is recognized in this art that the wheel bore and shaft surface interface is under a great degree of stress. This stress, in combination with other stresses generated by transient thermal or other unavoidable operating conditions, has been known to cause stress corrosion cracking indications in the hub section of the wheels. The precise mechanism which produces stress corrosion cracking is not fully understood, however it is believed that if the stresses at the wheel bore are kept at a minimum and the accumulation of water, condensed from the steam, is minimized and/or eliminated in that region, the probability of a stress corrosion cracking problem developing in that wheel will be reduced if not eliminated.
One prior art device which locks each wheel to the shaft utilizes a member protruding into a slot on the shaft. However, this relatively simple locking means significantly increases the stress concentration factor in that region of the wheel bore and consequentially increases the probability of a stress corrosion cracking problem in the wheel.